THAT'S WHY: WHY ARE SOME QUARTERS RED?

ANSWER: One of the world's most baffling mysteries! Surely you have been in the possession of a red quarter, yes? If you haven't, then just trust us: They're out there.Why would anyone want to color a quarter? And why do you find red quarters but not red nickels, or, except in the world of cliches, red cents?

The Why staff has been working slavishly on this puzzle for well over a year. We have repeatedly called the U.S. Mint. We have badgered the Federal Reserve. We have developed trustworthy sources at companies that make vending machines. We have contemplated the issue over long lunches at fashionable restaurants. The major break in the case came when we realized that the word for a person who has an unnatural obsession with coins - a coin hobbyist, in short - is "numismatist." This allowed us to then track down the obligatory hobby publication, Numismatic News, published in Iola, Wis. That publication's "Answer Man," Alan Herbert, put us out of our misery.

"I can tell you exactly what they are. These are so-called `house' or `shill' coins that a bar will use in a jukebox," he said. We had heard something like this before but never from so confident a source. The idea is that people won't feed the jukebox unless it's already throbbing. So the bar employees are given color-coded quarters to put in it. Later, when the machine is emptied by the jukebox vendor, it is obvious which ones were put into the machine by patrons and which ones by the bar employees. The bar keeps all of the red quarters and gets a share of the silver ones. Of course, this system doesn't always keep everyone honest, which is why those red quarters sneak their way into general circulation.

What makes this seem like a credible explanation is that it explains why the only red coins you see are quarters: That's all a juke box will take.

So why red? Why not green?

Because women don't paint their nails green. The coloring comes from nail polish, which sticks to a coin better than anything else.

QUESTION: Why are airline fares so crazy?

ANSWER: Plane fares make no sense to a human being but are utterly reasonable to a computer. To a human being, it is insane that a round-trip ticket costs LESS than a one-way ticket. To a computer, that's marketing genius. To a human being, it is bizarre that a flight of 400 miles costs twice as much as a flight of 800 miles. To a computer, that's "yield management."

Up until 1978 the government regulated the airline industry and there was nothing confusing about plane fares. All airlines had to charge the same amount. You picked your airline based on who had the better TV commercials, or who gave away the free playing cards.

Let's say you wanted to fly round-trip from Miami to Los Angeles in June 1976. There were only three options. A first-class ticket cost $568. A coach fare cost $374. An "excursion" fare cost $281, but you had to buy it 30 days in advance and could only fly during the middle of the week.

Then came deregulation. Today, the Miami-to-L.A. route has 82 separate round-trip fares. You got that right. Ten airlines run the route, and among them they charge 82 different prices as of late June 1990. For example, you could take American Airlines round-trip for $198. Or you could fly one way from Miami to Los Angeles on the same airline for . . . $360.

Even that is a discounted rate. The regular one-way coach fare from Miami to Los Angeles is $551, and the round trip is $1,102. This is for coach! You want first class: $1,640 round trip.

No longer are plane fares divided into coach and first class: Now, each seat on the plane can have its own assigned price. In the old days, when the industry was regulated, a $374 ticket meant that many people could never fly, and - just as bad - many rich people were paying less than they could afford. They weren't getting sufficiently soaked. Now comes the new system, wherein the trick is to stop the rich people and the expense-account travelers from buying those seven or eight seats set aside for tight-budgeted vacationers.

One way to do that is to pile advance-ticketing restrictions and no-refund provisions onto the cheap fares. Business travelers and rich snobs can't handle the loss of flexibility and will pay a higher fare. They like to buy tickets at the last minute and don't care if they cost $1,102 for seats in coach. The other customers for these tickets are grieving people who've just lost a parent.

Another trick is to stipulate that a cheap round-trip fare can only be used if the traveler stays in the distant city over a Saturday night. This staves off the business folk who want to be home by Friday night. And one-way fares are expensive because they are more likely to be purchased by an encyclopedia salesman making his rounds than by a vacationing granny.

What is happening here is quite simple: The airlines are charging people what they are willing to pay. It's the same trick that is used in the bazaars in Cairo. Before the merchant names a price, it must be ascertained what the customer has in his or her pocket. Yield management!

THE MAILBAG:

Tammy P. of unknown location asks how we know what the Milky Way galaxy looks like, even though we're inside it. Apparently Tammy has seen photos of the Milky Way and wants to know who or what took 'em. Those are probably pictures of another galaxy, such as Andromeda, which like the Milky Way is a spiral galaxy. Figuring out the shape of the galaxy isn't too hard, since we're out near one edge and can see almost the entirety of the thing on a clear night. After several centuries of staring at it we know it's turned on edge to us, and from a different angle would be shaped like a cinnamon roll.